1. Field of the Invention
The invention relates to line-narrowed excimer and molecular fluorine excimer and molecular fluorine laser systems, and particularly to a grating arrangement having a dielectric coating with comparatively high damage thresholds to radiation induced degradation which can be used as a line narrowing element in line narrowed excimer lasers, and also particularly to a line-narrowed laser resonator including a grating-prism element, or grism, preferably having a dielectric, antireflection (AR) coating on any transmitting surfaces and a dielectric, highly reflective (HR) coating on any reflection surfaces.
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, as well as the following generation of ArF-excimer laser systems operating around 193 nm. The ArF and KrF lasers have a broad characteristic bandwidth around 600 pm (FWHM). Vacuum UV (VUV) will use the F2-laser which characteristically emits two or three closely spaced lines around 157 nm.
It is important for their respective applications to the field of sub-quarter micron silicon processing that each of the above laser systems become capable of emitting a narrow spectral band of known bandwidth and around a very precisely determined and finely adjustable absolute wavelength. Techniques for reducing bandwidths by special resonator designs to less than 100 pm (for ArF and KrF lasers) for use with all-reflective optical imaging systems, and for catadioptric imaging systems to less than 0.6 pm, are being continuously improved upon.
For the application of excimer lasers as light sources for steppers and/or scanners for photographic microlithography, it is desired to have laser emission within a range that is much small than the natural linewidth which is approximately 300 to 400 pm for ArF and KrF lasers. The extent of the desired line narrowing depends on the imaging optics of the stepper/scanner devices. The desired bandwidth for catadioptic systems is less than around 50 pm, and for refractive optics it is less than around 0.8 pm. Currently, used systems for the KrF laser emitting around 248 nm have a bandwidth around 0.6 pm. To improve the resolution of the projection optics, a narrower laser bandwidth is desired for excimer laser systems of high reliability and very small bandwidth of 0.4 pm or less.
A line-narrowed excimer or molecular fluorine laser used for microlithography provides an output beam with specified narrow spectral linewidth. It is desired that parameters of this output beam such as wavelength, linewidth, and energy and energy dose stabilty be reliable and consistent. Narrowing of the linewidth is generally achieved through the use of a linewidth narrowing and/or wavelength selection and wavelength tuning module (hereinafter xe2x80x9cline-narrowing modulexe2x80x9d) consisting most commonly of prisms, diffraction gratings and, in some cases, optical etalons.
The line-narrowing module typically functions to disperse incoming light angularly such that light rays of the beam with different wavelengths are reflected at different angles. Only those rays fitting into a certain xe2x80x9cacceptancexe2x80x9d angle of the resonator undergo further amplification, and eventually contribute to the output of the laser system.
For the broadband excimer lasers such as the ArF and KrF lasers mentioned above, the central wavelength of the line-narrowed output beam may be tuned within their respective characteristic spectra. Tuning is typically achieved by rotating the grating or highly reflective (HR) mirror of the line-narrowing module.
Excimer lasers typically use planar gratings for narrow linewidth oscillation. The bandwidth xcex94xcex of the radiation is nearly given by, for a Gaussian line shape:
xcex94xcex≈(xcexxc2x7xcex94"THgr")/2xc2x7m1/2xc2x7tan xcex1xe2x80x83xe2x80x83(1)
xcexxe2x89xa1emission wavelength;
xcex94"THgr"xe2x89xa1divergence of the beam in front of the grating;
xcex1xe2x89xa1blaze angle of the grating;
mxe2x89xa1effective number of round trips of the beam in the laser resonator.
Typical grating substrates are made by Zerodure or ULE. A layer of epoxy is typically formed directly on the surface of the grating substrate, wherein the epoxy layer has a thickness between 12-40 xcexcm. The epoxy surface is then typically coated by aluminum, wherein the thickness of the aluminum coating is between 10-30 xcexcm.
A disadvantage of these types of gratings is that the aluminum absorbs more than 10% of the incident radiation power in the DUV spectral region within a very thin layer thickness. As a result, the gratings vary in their response to the exposure to high power laser beams by heating and aging. Moreover, nonuniform heating of the grating may substantially degrade its quality by, for example, distorting the wavefront of the retroreflected beam. Quality and long term stability of the optical components, as well as assembly, determines the behavior of the line narrowing unit under irradiation conditions in the laser cavity. It is desired to have a grating assembly that features relatively low absorption and a low degree of performance variance due to heating and aging. It is particularly desired to have a grating that does not effect the retroreflected beam such as by distorting its wavefront.
It is therefore an object of the invention to provide a line-narrowing package for an excimer or molecular fluorine laser system for producing an output beam at substantially less than 1 pm, and particularly less than 0.6 pm.
It is another object to provide a dispersive line-narrowing element for producing substantial line-narrowing in an excimer or molecular fluorine laser system that experiences relatively low absorption of incident laser radiation, such response variance due to heating and aging and effects of nonuniform heating are also relatively low.
In accordance with the above objects, a grating element is provided for use with a line-narrowing package of an excimer or molecular fluorine laser including a HR dielectric coating. This grating having the dielectric HR coating is configured to be disposed in a laser resonator to disperse an incident beam and to retroreflect the beam as a resonator reflector element.
Further in accordance with the above objects, a grating element is provided for use with a line-narrowing package of an excimer or molecular fluorine laser including a dielectric AR coating. This grating having the dielectric AR coating is configured to be disposed in a laser resonator in front of a resonator reflector, such as a HR mirror or partially reflective outcoupling mirror.
Further in accordance with the above objects, a grism is provided for use with a line-narrowing package of an excimer or molecular fluorine laser, preferably having a dielectric AR coating on any transmissive surface and a dielectric HR coating on any reflective surface.
In a first aspect, the grism may be configured to be disposed in the laser resonator with the grating surface facing the discharge chamber and serving as a HR reflecting resonator reflector. The grism according to the first aspect has a dielectric HR coating on its grating surface.
In a second aspect, the grism may be configured to be disposed in the laser resonator with the prism portion facing the discharge chamber and the grating portion serving as a HR reflecting resonator reflector. The grism according to the second aspect has a dielectric HR coating on the grating surface and a dielectric AR coating on the entry surface of the prism portion.
In a third aspect, the grism may be configured to be disposed in the laser resonator with the grating surface facing the discharge chamber and the rear surface of the prism portion serving as a HR reflecting resonator reflector. The grism according to the third aspect has a dielectric AR coating on the grating surface and a dielectric HR coating on the back surface of the prism portion. The grism may be disposed at a selected orientation with respect to the longitudinal cross section of the resonator, such that the prism portion of the grism may serve as a beam expanding prism.
In a fourth aspect, the grism may be configured to be disposed in the laser resonator with preferably the grating surface and alternatively the prism portion facing the discharge chamber, wherein neither the grating surface nor the entry/exit surface of the prism portion serves as a HR reflecting resonator reflector. The grism according to the fourth aspect has a dielectric AR coating on each of the grating surface and the entry/exit surface of the prism portion. In use, the grism is preferably disposed in front of a HR resonator reflecting mirror or partially reflecting outcoupling mirror. The grism may be disposed with the prism portion facing the discharge chamber and at a selected orientation with respect to the longitudinal cross section of the resonator, such that the prism portion of the grism may serve as a beam expanding prism.
In a fifth aspect, the grism may be configured to be disposed in the laser resonator as an output coupling element with either the grating surface or prism portion facing the discharge chamber, and, in either case, either the grating surface or prism portion serving as a partially reflecting resonator reflector surface. In this case, the surface that serves as the partially reflecting resonator reflector surface is partially reflecting and may be uncoated or coated, while the other surface has a dielectric AR coating on it.